September 1998, Issue 98

PIC'Spectrum
Audio Spectrum Analyzer

PIC’SPECTRUM HARDWARE

Now that we have a good microcontroller, let’s look at the PIC’Spectrum hardware shown in Figure 4.

A small power supply, which is built around U1 (78M05), generates a clean 5 V from a standard 9-VDC power supply (the total power consumption is around 100 mA, which is mainly 50 mA for the PIC and 50 mA for driving the three 75-W video outputs). The LED D1 indicates powerup and generates a pseudoground 1.9-V level used by the analog section.

The analog input signal (from an onboard electret measurement microphone or a line input jack) is amplified and low-pass filtered down to 10 kHz by four operational amplifiers (U2, LM324). A potentiometer lets you adapt the amplifier gain to the ambient sound level and serves as an on-off switch. The output signal, centered on the 1.9-V pseudoground level, connects directly to one of the PIC’s analog inputs.

The PIC processor is clocked by a 32-MHz crystal (I wasn’t able to find a 33-MHz crystal in time). As usual for these frequencies, this crystal is a 3´ overtone model.

I needed a damper circuit (L1/C8) to select the correct resonant frequency. Without it, the crystal would oscillate on its fundamental frequency, and I’d end up with a 10.66-MHz clock!

Two switches (K1 and K2) control the current mode (run or hold) and the scaling of the display (linear or logarithmic). They connect directly to RB6 and RB7 because the PIC has selectable internal pullups.

The VGA-compatible video-output connector is directly driven by the PIC. The horizontal and vertical synchronization signals are TTL compatible, so there’s no problem there. For the R, G, and B lines (0–2 V/75 W), a 150-W series resistor does an adequate 5-V to 2-V/75-W adaptation, thanks to the high power capacity of the PIC outputs (20 mA/line, 100 mA total for ports A and B).

The 8-pin header J4 handles in-circuit serial programming. Four additional output pins (including one UART output) are connected to a debug header. This provision is useful during the debug phase because it enables you to send debug information to a serial terminal to find out what’s happening inside the box when you don’t have an in-circuit emulator.